This invention relates to a data decoder; and it relates, more particularly, to a decoder which operates to utilize probabilistic information in a received signal for facilitating the extraction of information therefrom.
In a data transmission system, the character of a fading transmission medium per se and the character of noise and interference, which are likely to be superimposed on a signal during transmission through that medium, tend to make it difficult to extract information from a received signal. For example, intersymbol interference tends to blur information representative states of adjacent data bits; and an electromagnetic noise burst can combine with a data signal to change partially, or completely, the originally transmitted binary information state of the data signal portion affected. Various techniques are known in the art for operating in the presence of these conditions to extract synchronizing information and/or reasonably intelligible data information from received signals. Some examples of those techniques are identified below; but in a fading channel, and particularly when FM modulation is used, these all employ so little of the available probabilistic information that error rates are excessively high and thus impede rapid, useful, information throughput.
A U.S.A. Pat. No. 3,341,658 to Kaneko discusses the use of a maximum likelihood technique for synchronizing signal recovery vis a vis data message information recovery. For that purpose, there are employed a matched filter, a waveform generator representing a log function of a priori probability of the synchronizing time point, a combiner for adding the waveform output of the generator to a correlation filter output to enhance the correlation output at a time when the probability of the synchronizing time point occurrence is large, and a detector for indicating when the combined output is above a fixed detection level.
A U.S.A. Pat. No. 3,633,107 to Brady shows an adaptive signal processor for diversity radio receivers and using a transversal filter to process analog signals in a space diversity system wherein the summed tap outputs are fed back to control tap multipliers for tap signal weighting.
A U.S.A. Pat. No. 4,029,900 to E. J. Addeo, shows an example of a timing recovery system for noisy, fading, multipath channels of a mobile radiotelephone system. The recovery system of the patent employs both analog techniques, to derive approximate bit phase information, and digital techniques, to derive from the approximate information more precise phase information to be used in digital logic for synchronizing word detection.
Another U.S.A. Pat. No. 4,242,755 to Gauzan deals with an arrangement for decoding digital signals and in which multiple samples of each incoming bit signal are taken. The samples are processed to distinguish between binary ONE and binary ZERO bits by detecting the relative numbers of samples in each such bit state during a single bit time.
It is also known in post-detection maximal-ratio signal combining arrangements to employ signal weighting in the output of each received signal branch. The weighting is accomplished by a measure of the output signal-to-noise ratio, as taught by W. C. Jakes at pp. 390-395 in Microwave Mobile Communications, published 1974 by John J. Wiley and Sons, Reading, Mass.